There is a growing consensus that the collection of solar energy and its conversion to electrical energy by means of photovoltaic devices should be included in the energy mix of the near future. The commercialization of photovoltaic devices depends on technological advances that lead to higher efficiencies, lower cost, and stability of such devices. The cost of electricity can be significantly reduced by using solar modules constructed from inexpensive thin film polycrystalline semiconductors such as copper indium diselenide (CuInSe.sub.2 or CIS) or cadmium telluride (CdTe). Both materials have shown great promise, but certain difficulties have to be overcome before their commercialization.
Thin-film photovoltaics have two important advantages that offer the hope of achieving truly low-cost electricity production in the near future. The first key to using thin-film photovoltaics is that the material costs remain a small part of the total cell cost and the thin-film coating on large substrates can be obtained in sufficiently large volumes and at sufficiently low costs. The second key to using thin-film photovoltaics is that they hold the promise of being mass-produced in automated processing lines. Two of the leading candidates among thin film solar cells are cadmium telluride and copper indium diselenide. These thin film solar cells are developed with very simple and inexpensive processing techniques such as closed space sublimation (CSS), evaporation, and sputtering, on inexpensive window glass which also acts as a structural support for the final modules. Besides the reduced material cost, simpler and less expensive processing and simpler handling allow for a significant reduction in cost over crystalline silicon cells. However, problems also exist, slowing the thin film solar cell technology in its development and commercial use. A generic problem that is associated with all thin film solar cells is their low conversion efficiencies. The thin-film semiconductor layers are polycrystalline in nature. The inherent grain boundaries introduce regions of increased disorder and segregated impurities in large densities, resulting in a loss of photogenerated carriers due to increased recombination rates. Nevertheless, there has been continuous progress in achieving higher thin-film solar cell efficiencies during recent years despite the poor photovoltaic properties of compound materials developed by low-cost processing methods.
Gallium has been used beneficially in CIS devices, resulting in a copper-indium-gallium-diselenide (CuIn.sub.x Ga.sub.1-x Se.sub.2 or just CIGS) structure. Gallium helps to improve the adhesion properties of the CIS films to the substrate. Also, by engineering the band gap of the CIS through Ga incorporation in the space charge region, device efficiencies have been improved further. CIGS laboratory solar cells with efficiencies in the 15-17% range have been reported by several organizations.
However, the deposition techniques used to achieve these levels of performance are either complex codeposition or a two-step process of deposition of a precursor followed by selenization with H.sub.2 Se gas. Each of these methods has shortcomings relative to manufacturing scale-up. The codeposition technique requires tight control of the elemental sources and is quite complicated. The two-step process relaxes this issue but adds the difficulty of dealing with H.sub.2 Se gas, a class A toxic gas, the use of which significantly adds to the complexity of using this method. Attempts at simplifing CIGS manufacturing techniques, for example, by depositing a precursor of one or more of copper, gallium, selenium, and indium and then "selenizing" that precursor by heating it in the presence of a selenium flux, have not resulted in conversion efficiencies as high as ultimately expected from such techniques.
Also, ideally, an absorber layer of a photovoltaic device must have a good bulk layer (e.g., values of space charge width of about 0.5 .mu.m and a minority carrier diffusion length of 1-2 .mu.m) and must have a good absorber surface (e.g., low defect density resulting in a recombination lifetimes preferably on the order of 5.times.10.sup.-9 seconds or greater). Many prior art techniques focus on a manufacturing process that is an apparent compromise between achieving a good bulk layer and achieving a good absorber surface. The result is that neither the surface properties nor the bulk properties are optimized; the process is too complex to simultaneously optimize both properties at the current state of technology.